Class Notes (839,329)
Canada (511,271)
HMB200H1 (140)

HMB200 2014 Lecture 21.pdf

5 Pages

Human Biology
Course Code
John Yeomans

This preview shows pages 1 and half of page 2. Sign up to view the full 5 pages of the document.
  Lecture  21:  Brain  Growth  and  Neural  Connections     Postnatal  Brain  Growth   -­‐ at  birth,  don’t  want  too  big  of  a  brain  weight  because  a  big   head  is  hard  to  get  out  from  birth  canal   -­‐ after  birth,  neurons  become  larger;  brain  weight  goes  up,   volume  of  each  neurons  gets  bigger   –  growing  more   dendrites  to  contact,  growing  lots  of  myelin  (last  process  that   continues  after  birth),  growing  many  more  s ynapses/neuron   -­‐ maturing  of  neurons,  then  connections,  then  myelin  after   birth   -­‐ the  number  of  neurons  is  usually  established  already   -­‐ neurons  have  chemicals  that  can  differentiate  their  differentiation     Neurotropic  Factors   • Proteins  expressed  in  specific  neurons   • Cell  survival  is  stimulated  by  neurotropic  factors   • Improve  survival  of  cells  and  synaptic  connections,  neurite  growth,  and  plasticity  (changes  in  synaptic  strength).   • Nerve  Growth  Factor:  Sympathetic  &  chol inergic  basal  forebrain  neurons   • First  neurotropic  factor  discovered   • Took  extracts  of  sympathetic  neurons,  applied  it  to  other  sympathetic  neurons,  you  can  grow  the   sympathetic  neuron  to  grow  in  a  dish  =  neural  growth  factor   • Extracted  ONE  protein  from  other  sympathetic  ganglia,  when  the  protein  is  appli ed  to  other  sympathetic   neurons,  this  neuron  suddenly  grows  lots  of  axons  and  dendrites   • One  protein  stimulated  nerve  growth   • When  this  nerve  growth  factor  was  applied  to  other  neurons,  there  was  no  effect   à  selective  for   sympathetic  neurons   • Very  specific  to  which  neurons  they  activate   • BDNF,  NT3,  NT4/5.   • Can  extract  growth  factors  from  brain   –  BNDF  (brain  derived  neurotropic  factors)   • Can  extract  growth  factors  from  glia  –  glial  factor   • Each  derived  from  cultured  cells  (sympathetic  neurons,  brain,  glia)  and  act   on  special  receptors  (Trk  and  p75)   • Act  by  way  of  neurotropic  receptors  (Trk  receptors,  p75  receptors)   • Neurotropic  factors  are  working  in  the  substrate,  can  be  released  by  different  cells   • BDNF  mainly  found  in  microglia   –  released  by  microglial  neurons,  stim ulates  receptors  on  sympathetic   neurons,  then  can  turn  on  growth  (stimulate  more  processes,  more  connections,  bigger  and  better   synaptic  connections)   • Chemical  induction  factor  that  allows  them  to  survive  or  not     Synaptic  Connections   • Neurons  matured  earlie r  can  make  axons  earlier  and  go  longer  distances   –  primary  cells  (big   output  cells)   • As  growth  cone  reaches  target,  they  start  making  synapses   • Once  attracted  by  some  tissue,  it  makes  hundreds  of  terminals  (one  axon  can  have   hundreds  of  terminals  on  lots  of   different  cells)   • Many  synapses  are  formed  initially.   • Proliferation  of  terminals  at  the  tip   • Most  of  these  synapses  are  eliminated  by  competition  between  inputs.   • Over  time,  diffuse  extra  connections  die  off   –  only  the  point-­‐for-­‐point  connections   survive  à  competitive  process   • Competition  involves  chemical  signals  (e.g.,  trophic  signals)  and  electrical  signals  (activity -­‐dependent  plasticity).   • More  inputsàmore  dendritesàmore  synapses.       Brain  Systems  for  Studying  Connections   • Crossing  fibres  of  ventral  spinal  co rd.  (netrin  attracts,  slit  repels  axons)       • Crossing  fibers  grow  by  crossing  midline  of  spinal   cord  by  being  attracted  by  netrin  at  first,  then   repelled  by  slit  as  it  leaves   • Retinotopic  maps  (2D)  in  the  tectum  of  the  superior   colliculus,  thalamus  and  cortex   • If  you  take  a  retina,  and  put  it  near  the  Colliculus,   those  retinal  axons  will  grow  out  towards  the   Colliculus  à  attracted  to  the  Colliculus   • How  is  the  2D  map  formed  on  the  Colliculus?   • As  they  go  into  the  Colliculus,  they  go  into  the   outer  layers,  then  segregate  so  that:   •  the  nose  end  and  back  end  of  the  retina   go  to  one  part  of  the  tectum   • and  the  dorsal  side  and  ventral  side  of  the   retina,  goes  to  the  other  part   • there  is  a  chemical  inside  the  Colliculus  (ephrin)   • there  is  a  2D  gradient  of  a  particular  prote in   • ephrin  A:  up  and  down;  ephrin  B:  side  to   side   • the  density  of  ephrin  is  located  so  that  there  is  low   ephrin  on  one  end,  and  high  ephrin  on  the  other   • nasal  neurons  go  to  high  ephrin  area ,  and   temporal  axons  go  to  the  low  ephrin  area   • the  growing  axons  read  the  density  of  ephrin  to   determine  where  they  terminate   • if  you  cut  out  half  of  the  retina,  they  still  maintain  this  gradient     • chemical  influences  how  they  approach,  and  how  they   avoid  to  form  connections   • efferent  levels  (up  &  down,  side  to  side)  determin es  which   retinol  fibers  connect  with  which  superior  Colliculus  area   • a  simple  chemical  gradient  that  determines  how  the   retinotopic  map  is  formed   • retinol  neurons  respond  to  this  gradient  to  form   connections,  which  makes  the  retinotopic  map   • Mutant  frog  with  3  eyes   rd • By  adding  a  3  eye  =  changes  in  connections   • Inputs  from  2  eyes  form  separate  columns  in  the   contralateral  tectum  –  retinotopic  maps   • When  there  is  3  eyes  innervating  the  tectum,  it  messes  up   the  maps  à  different  organization  within  the  left   Colliculus     o This  is  experimental  embryology:  change  the  embryo  by   changing  the  chemicals,  the  number  of  inputs,  where  those  inputs  enter  =  alter  way  the  brain  is  organized   • Mutant  flies  and  mice.   • In  experimental  embryology:  single  chemicals  can  change  the  growth,   single   genes  can  change  the  growth   à  influences  connections  and  behaviour   • Cerebellar  mutants,  weaver,  reeler.       Critical  Periods   • growth  in  Colliculus  occurs  early  in  development   à  before  animal  is  born   • specific  synaptic  connections  can  occur  prenatally  (ea rly  development  in   the  retina,  thalamus,  spinal)  or  later  in  time  (development  of  cortex  occurs   later)   • Synaptic  connections  formed  at   different  times  in  different  brain  areas.  These   times  are  critical  for  functional  development.   • Most  of  the  old  areas  in  th e  brain  occurs  prenatally   • Most  of  the  new  areas  (cortical  development)  occurs  postnatally   –  where   later  connections  of  the  cortex  occurs       • Lower  brain  earlier,  and  much  more  fixed  (occur  prenatally)  –  these  connections  are  determined  by  gene   expression  and  chemical  factors  =  more  fixed   • Same  in  all  animals   • Later  on,  these  are  influenced  by  neural  firings     • Neural  firings  occur  later  in  development  (especially  in  the  cortex  after  birth)   • Cortex  forms  after  birth,  connections  are  influenced  by  outside  environment.   • Cortical  connections  are  formed  by  a  new  cortical  chemical  =  NMDA  (glutamate  chemical)   • Cortical  connections  after  birth  are  influenced  by  firings  of  glutamate  neurons  (influenced  by  NMDA   receptors  after  neurons  fire  –  influenced  by  internal  activity  within  the  cortex)   • NMDA  is  important  for  memory,  cortical  development,  changing  throughout  lifetime  (synaptic  plasticity)   • Cortex  keeps  changing  throughout  life,  and  influences  thalamic  maps.   • Synaptic  plasticity  is  important  for  memory   –  synaptic  connections  are  constantly  changing       Electrical  Activity  and  Growth   • Retina:  Prenatal  waves  of  electrical  activity  in  each  eye  independently.   • Right  eye  active  first,  then  left   • Left  eye  has  a  brief  period  of  electrical  activity   • The  2  eyes  have  internal  factors  that  produ ce  electrical   exc
More Less
Unlock Document

Only pages 1 and half of page 2 are available for preview. Some parts have been intentionally blurred.

Unlock Document
You're Reading a Preview

Unlock to view full version

Unlock Document

Log In


Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.